Review pubs.acs.org/acscatalysis
Graphene-Based Metal-Free Catalysts for Catalytic Reactions in the Liquid Phase Pei Tang,† Gang Hu,‡ Mengzhu Li,† and Ding Ma*,† †
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Israel Chemicals Limited, Shanghai 200021, China
‡
ABSTRACT: The building blocks of the well-defined and easily modified carbon-bond structure of graphene sheet units bring unique characteristics to carbon materials. Especially when these carbon materials are nanosized, their tunable electronic states, giant conjugated structures, oxygen groups on the defects of graphene sheets, and various dopants all help make the materials active in liquid-phase catalytic reactions. In this mini-review, we mainly focus on the liquid-phase reactions catalyzed by carbon nanomaterials, and the related mechanisms are discussed on the basis of their structure fundamentals. In the final section, we also provide our perspectives on this rapidly developing carbocatalysis field. KEYWORDS: carbocatalysis, graphene, catalysis, metal-free
1. INTRODUCTION Over the past 6 years, more than 30 new catalytic systems utilizing graphene-based carbon nanomaterials as direct catalysts have been developed as a means to meet the demand of sustainability. Over 90% of these practical chemical engineering processes are catalytic reactions, most of which are based on metal/metal oxide catalysts, and especially noblemetal catalysts, such as Pt, Rh, and Pd, are utilized in 60% of the processes.1−3 However, the limited reserves, sensitivity to poisoning, and high price of noble metals prevent wide applications of noble-metal catalysts in chemical processes. In addition, the possible noble-metal residues in the reaction systems may hinder the subsequent purification processes.4 Therefore, developing catalysts from sustainable materials could be a smart way to overcome these shortcomings.5 Carbon nanomaterials, which can be produced from biomass, lowcarbon hydrocarbons, and small organics, are good candidates in this field because of their large surface area, outstanding thermal stability, and potential for various chemical modifications. These metal-free catalytic processes, which utilize carbon materials as direct/active catalysts rather than inert catalyst supports, are termed “carbocatalysis”.6−9 The knowledge of carbon nanomaterials from nanochemistry greatly promotes the development of this emerging research field in providing kinds of carbon materials. Carbon nanomaterials such as fullerenes, carbon nanotubes, graphene, and their derivatives have all been found to be catalytically active in certain reactions. These carbon materials are all constructed from well-defined and easily modified graphene sheets. These carbon-bond structures are the key factor to determine the catalytic activities of these carbon materials. In particular, when these carbon materials are nanosized, their tunable electronic states, giant π-conjugated © 2016 American Chemical Society
structures, oxygen groups on the defects of graphene sheets, and various dopants can all be active sites in catalytic reactions. Carbocatalysis is the study of chemical reactions using carbonaceous materials as catalysts. These catalytic materials are prepared and used in powder or monolith form, and the reactions are therefore heterogeneous. In contrast, organocatalytic systems usually concern small organic molecules, and the discussion in this field belongs to homogeneous catalysis.7,10 It is worth noting that carbocatalysis has been known for decades, since the first discovery of catalytic activities of carbon materials.11 In 1925, Rideal and Wright12 discovered that charcoal can catalyze the oxidation of oxalic acid, which was among the first reports of carbocatalysis. However, 45 years earlier, carbon materials were already found to be able to catalyze the conversion of halogenated hydrocarbons.11 It is easy to understand that most of these catalytic systems at that time were based on traditional carbon materials, such as activated carbons, graphite, and carbon blacks. The work of Schlögl13 and Su14 on oxidative dehydrogenation reactions brought the research interest from traditional carbon materials to carbon nanomaterials and inspired the research progress in this field. Later, Bielawski and co-workers first provided the opportunity to apply these graphene-based carbon materials in the area of catalytic reactions in the liquid phase.15 Scientists can now prepare carbon nanomaterials exhibiting better catalytic activities than their traditional counterparts in most of the carbocatalytic systems. Meanwhile, it should be noted that many of these carbocatalytic systems still cannot exceed Received: June 13, 2016 Revised: August 30, 2016 Published: September 2, 2016 6948
DOI: 10.1021/acscatal.6b01668 ACS Catal. 2016, 6, 6948−6958
Review
ACS Catalysis the metal/metal oxide catalyst systems in catalytic activity, especially for multielectron processes. In this sense, even trace amounts of metal residues in carbon nanomaterials may significantly influence the catalytic activities and even the kinetic profiles of these reactions.6,7,9 It is of vital importance for researchers to discriminate the contribution of metal residues when a new carbocatalytic system is investigated. Accordingly, several possible strategies concerning this topic will be discussed herein. In this review, we mainly focus on reactions catalyzed by graphene-based nanomaterials (graphite oxides, graphene oxides, graphene, and their derivatives) in the liquid phase, as reactions in the gas phase have been well-reviewed by others and the applications of carbon nanomaterials in electronics, electrodes, and photocatalysis are also discussed in other reviews.16−22 First, we briefly introduce the structure and preparation of graphene-based carbon nanomaterials in order to exhibit the correlation between the fundamental structure and the catalytic activities of carbon nanomaterials. Then the carbocatalytic reactions and related mechanisms are discussed in categories of oxidation reactions, reduction reactions, and coupling reactions. Finally, in the Perspective we provide several strategies to evaluate the influence of metal residues and also our viewpoints of the future of carbocatalysis.
Scheme 1. Schematic Diagram of Graphene-Based Carbon Materials and Their Possible Oxygen Contents, Colors, and Surface Areas
interlayer oxygen atoms of graphite oxides are removed, the densely stacked structures of graphitic layers collapse to form graphene oxide materials with a higher surface area (∼500 m2/ g) but a lower oxygen content (10−20 A.R.%). In our point of view, those materials termed as graphene oxides, reduced graphene oxides, pyrolytic graphene oxide, and chemically converted graphene in various publications are virtually the same compound and are denoted as graphene oxides (GO) in this review. When intense reduction processes, such as hightemperature (>800 °C) annealing treatments, are employed to further reduce these graphene oxides to products with an oxygen content of less than 5 A.R.%, the obtained products are denoted as graphene (G). It is worth noting that such materials are sometimes denoted as reduced graphene oxides even after intense reduction processes. Despite the inconsistent terminology for graphene derivatives, we suggest that they be categorized on the basis of their oxygen content. In parallel with the oxygen content, the thickness (i.e., the number of stacking layers of carbon atoms) is also an important parameter to address for the study of graphene materials. The graphene materials (graphene oxides and graphene) in Scheme 1 usually have a surface area of ∼500 m2/g, which is much lower than that of ideal monolayer graphene (∼2600 m2/g), as they are chemically converted from graphite oxides. These graphene materials are mainly constituted of two to five ideal graphene monolayers, which is also consistent with the definition that two-dimensional carbon materials consisting of less than 10 monolayers can be called graphene. In some publications, these graphene oxides and graphene materials are also called layered carbon.37,40 2.2. Synthesis. Both bottom-up and top-down methods have been utilized to prepare these graphene-based nanomaterials. Chemical vapor deposition (CVD) is the most commonly used bottom-up approach. Graphene materials thus prepared are of good quality but obtained in poor yields of several milligrams per day, which could not meet the demand of graphene materials in liquid catalysis systems (on the scale of a few tens of milligrams for each reaction).41 Producing carbon materials from small organic molecules or biomass is also possible, yet these materials usually have low levels of graphitization and are not within the scope of discussion in this review.42−44 The graphene-based nanomaterials used in liquid catalysis are prepared by top-down methods, which are chemical conversion processes. In these processes, Hummers’ method, which utilizes H2SO4, KMnO4, and NaNO3 as oxidants, is the most commonly used one.45 Some other approaches, such as Brodie’s method and Staudenmaier’s method, are also
2. FUNDAMENTAL STRUCTURE AND SYNTHESIS OF GRAPHENE-BASED NANOMATERIALS 2.1. Structure. The main reason to explore new catalytic systems by utilizing carbon nanomaterials as direct catalysts is that they possess at least four types of active sites as a consequence of the building blocks of well-defined and easily modified graphene sheets. In this section, we mainly focus on the definition of these structural characteristics of graphenebased nanomaterials that may have a correlation with their catalytic activities. The ideal graphene sheet, as the basic building block of carbon materials, is formed by a single layer of carbon atoms, each of which is bonded to three neighboring carbon atoms. Such sheets can form all types of carbon materials, such as fullerenes, carbon nanotubes, graphite, and activated carbons.23 Researchers of graphene-based nanomaterials tend to focus only on two-dimensional layered carbon materials, including graphite oxides, graphene oxides, graphene, and their derivatives with doping or chemical modification, and the term “graphene-based carbon materials” mainly indicates these two-dimensional carbon materials. Furthermore, one may feel puzzled in literature reviews by the terms graphite oxides,24−26 graphene oxides,27−29 reduced graphene oxides,30,31 pyrolytic graphene oxides,31,32 chemical converted graphenes,33,34 and graphene.27,35 Confusion with these terms may lead to misunderstanding when the catalytic activities of these graphene-based carbon materials are compared. We propose to discriminate these materials on the basis of their oxygen contents, as shown in Scheme 1. Graphite oxides are the products of direct oxidation of graphite with an oxygen content of more than 40 atom ratio % (A.R.%). These oxygen atoms are inserted between the graphene layers of graphite as a result of stepwise oxidation processes. Such graphite oxides usually have a surface area of ∼30 m2/g and can be further processed to obtain graphene oxides by removal of oxygen atoms from the interlayer galleries. The methods include sonication,36 pyrolysis,32,37 and mild chemical reduction (utilizing NH2− NH2,31,36 NaBH4,38 and alcohols39 as reductants). When the 6949
DOI: 10.1021/acscatal.6b01668 ACS Catal. 2016, 6, 6948−6958
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ACS Catalysis Scheme 2. Schematic Diagram of Active Sites of Graphene-Based Nanomaterialsa
a
Reproduced from ref 6. Copyright 2013 American Chemical Society.
available.27,46 Although different oxidants are used, the methods follow the same stepwise oxidation principle.27,44,45. These socalled modified Hummers’ methods utilize larger amounts of oxidants and longer reaction times than the original one reported in 1958 in order to obtain graphite oxide with higher oxygen content.45 The post-treatment methods to modify graphene-based nanomaterials are also of vital importance. Most of the doping processes of such materials are fulfilled by post-treatments.47−49 It is worth noting that such processes are sometimes regarded as CVD processes, although this is inappropriate and misleading as such processes do not go through the intermediate of chemical vapor and should be more appropriately categorized as post-treatment or annealing. There have been reports on the production of porous activated carbon from natural graphite or biomass, which were also applied on graphene-based materials to prepare porous graphene or nanomesh materials.50,51 Building covalent bonds on these carbon materials by chemical modification can also generate excess active sites on graphene-based nanomaterials.52 There are at least four types of active sites in these easily modified graphene-based carbon materials, namely, tunable electronic states, giant π-conjugated structures, oxygen groups on the defects of graphene sheets, and various dopants, as shown in Scheme 2. Tunable Electronic States. The first influence of the process of converting traditional carbon materials to nanosized carbon nanomaterials is the enlarged surface area and subsequently the tunable electronic state of the carbon material.53 The superfluous electronic states of graphene materials on zigzag edges make them excellent catalysts in the reduction of nitrobenzene. Furthermore, heteroatoms in graphene sheets could also alter their electronic states to promote the catalytic activities in liquid catalysis.4,37 Giant π-Conjugated Structures. The unique giant πconjugated structure of carbon materials is a result of delocalized electrons of carbon atoms. This unique feature brings natural interactions between aromatic compounds with these graphene-based carbon materials and greatly facilitates the adsorption/activation of aromatic compounds on graphenebased carbon.34 This also leads to a phenomenon that more than 70% of the substrates of these reactions catalyzed by graphene-based nanomaterials are aromatic compounds.
Oxygen Functional Groups on Defects of Graphene Sheets. There are at least five types of oxygen functional groups hanging on graphene sheets of graphene-based carbon materials. These are mainly carboxyl (−COOH), carbonyl (−CO), hydroxyl (−OH), ketone (−CO), and epoxy (−C−O−C−) groups. These oxygen functional groups bring at least four categories of catalytic capabilities to carbon materials: (1) as acidic groups to promote acid-catalyzed reactions;54,55 (2) as intermediates to react with oxidants and transfer oxygen atoms to substrates;15 (3) as nucleophilies to promote some coupling reactions; and (4) as defects/holes on graphene sheets by impairing the perfect π-conjugated structure so as to promote the catalytic reactions.56 Various Dopants. Heteroatoms of N, B, P, and Se can be incorporated into the lattice of graphene sheets to form socalled tribonds with carbon atoms to obtain doped graphene materials. Besides, other heteroatoms such as S, F, and Cl can also form covalent bonds with carbon materials,4,57 although they do not form tribonds in the graphene layer. Functional groups such as −SO3H groups can also be grafted onto graphene sheets by organic reactions.52
3. OXIDATION REACTIONS Amine, alkene, alkane, and aromatic compounds were found to be able to undergo conversion to the corresponding oxygenated chemicals catalyzed by graphene-based nanomaterials. The observation of oxygen adsorption on the surface of charcoal led to the discovery of the oxalic acid oxidation reaction catalyzed by charcoal.12 It is also widely accepted that graphite and activated carbon can catalyze the decomposition of 4chlorophenol by H2O2. It is proposed that controlled activation of H2O2 by the oxidation groups on the carbon materials dominates this catalytic process.58−60 The nanosized and oxygen-group-rich graphene-based carbon materials provide a more resourceful platform for the development of carbocatalyst systems. Bielawski’s group was among the first to explore liquid-phase reaction systems catalyzed by graphene-based nanomaterials. In 2010, they discovered that these graphite oxides could catalyze the oxidation of benzyl alcohol to benzaldehyde by air.15 As shown in Scheme 3, when 200 wt % graphite oxide is used as the catalyst, more than 90% conversion of benzyl alcohol and more than 95% selectivity of benzaldehyde are obtained. By a control experiment in a 6950
DOI: 10.1021/acscatal.6b01668 ACS Catal. 2016, 6, 6948−6958
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ACS Catalysis Scheme 3. Oxidation of Benzyl Alcohol and Stilbene Using Graphene Oxide15
Scheme 4. Proposed Mechanism for the Oxidation of Benzene on Graphene Oxidea
a
Reproduced with permission from ref 34. Copyright 2013 Royal Society Chemistry.
nitrogen atmosphere, they also proved that this process utilizes air to oxidize these compounds, and they found that the graphite oxide can be reused. These catalytic systems have also been expanded to oxidations of other alcohols and olefins. Their following work showed that these oxidation products could be autocoupled to chalcone.61 According to density functional theory (DFT) calculations, the transfer of hydrogen atoms from the organic molecules to the graphite oxide surface and the ring-opening and reoxidation processes of epoxide groups on the graphite basal plane are key steps to complete the catalytic process.62 A similar catalytic system to convert 5hydroxymethylfurfural to 2,5-diformylfuran catalyzed by GO was reported by Hou and co-workers.63 Besides these traditional heating treatments, a sonication treatment was applied to promote such reactions.64 Bielawski and co-workers also developed a system for the oxidation of sulfides to sulfoxides with graphite oxides as the sacrificial agent. In that work, the sealed reaction environment and the deoxygenation of graphite oxides indicated that this process is not a real catalytic one.65 A similar oxidation system using graphite oxides as sacrificial agents to promote lots of oxidation reactions, including the oxidation of olefins to their respective diones, methylbenzenes to their respective aldehydes, diarylmethanes to their respective ketones, and various dehydrogenations, was also reported.66 A new approach that utilizes graphite oxides as oxidizing agents to promote the oxidative aromatization reaction of 1,4-dihydropyridines was also realized.67 Ma and co-workers demonstrated that GO can catalyze the oxidization reaction of benzene to phenol with the addition of H2O2. In this reaction, 18% conversion of benzene was obtained with a selectivity for phenol of about 99% after a reaction time of 8 h.34 It was proposed in the paper that the activation of both benzene and H2O2 are important in this process. It was proven by a temperature-programmed desorption (TPD) process that benzene has a stronger interaction with GO than with graphite and graphite oxides. The activation energy for hydrogen peroxide activation is about 47 kJ/mol, which is suitable for this oxidation reaction. Scheme 4 shows the fact that the cooperation/balance of the activation of benzene and hydrogen peroxide is the key to achieve this reaction. The GO catalyst did not show obvious catalytic activity loss in seven cycles. Furthermore, such GO catalysts are much more active in the phenol oxidation reaction than multiwalled carbon nanotubes, which created only about a 2% conversion of benzene in 3 h.56 Another example is the GOcatalyzed oxidation of tetralin to tetralol and tetralone.44 In this reaction, oxygen in the atmosphere was used to oxidize the tetralin with a conversion of 20%, and the selectivities for tetralol and tetralone reached 92% after reaction for 24 h. Peng and co-workers reported an important catalytic oxidation conversion of cyclohexane to cyclohexanol, cyclohexanone, and adipic acid with a series of carbon materials.68
Micrometer-sized diamond (μ-D) and nanosized diamond (nano-D) graphite powders in 325 mesh (G325) and 4000 mesh (G4000) and graphene oxides were able to catalyze the oxidation of inert cyclohexane to the corresponding oxygenated chemicals with 1.5 MPa oxygen (Table 1). The researchers also found that the sp2-hybridized carbon materials are more active in this reaction because of their better catalytic capabilities in decomposing the key intermediate C6H11COOH. Besides the investigations of as-prepared carbon materials, there have been some reports on activation treatment of graphene-based carbon materials in order to obtain improved catalytic activities. Loh and co-workers reported an oxidative coupling reaction of primary amides to give imines utilizing air, and interestingly, they showed improved catalytic capabilities by bringing the yield up from 44% to 89% after a sequential base and acid treatment to dig holes in the graphene oxide. They proposed that the edges in defect sites of graphene sheets and the grafted carboxylic groups were important in catalyzing this reaction.51 Besides the base and acid treatment, the researchers also demonstrated that ZnCl2 and CO2 are also efficient activators for treatment of graphene-based carbon materials, particularly when they are used in combination. Wang and co-workers discovered that graphene oxides activated by both ZnCl2 and CO2 are more active in the oxidative degradation of methylene blue and can adsorb more methylene blue than the unactivated ones.50 These activation methods mentioned above are original from these prepared processes of activated carbon and are universal to create defects for carbon materials. These catalytic results reported by Nishina and co-workers also released a fact that besides these oxygen groups, the defects in graphene sheets also greatly influence their catalytic capabilities.69 Ma’s group explored N-doped graphene materials as catalysts in selective oxidation reactions and found that the N-doped graphene materials could activate the benzylic C−H bond of ethylbenzene and convert the ethylbenzene to acetophenone using tert-butyl hydroperoxide. A >90% yield of acetophenone was obtained after a 24 h reaction. The amount of N atoms in the graphene materials was found to significantly influence the catalytic performance. Detailed analysis of the chemical environment of N atoms in the doped graphene materials showed that the graphitic N atoms dominate this catalytic process. X-ray absorption spectroscopy (XAS) (Scheme 5) indicated that the tert-butyl hydroperoxide (oxidant) and ethylbenzene (reductant) just changed the chemical status of carbon atoms while the N atoms retained their original chemical status during this catalytic cycle. DFT calculations further revealed that the graphitic N atoms influence the neighboring carbon atoms by altering the electronic partial 6951
DOI: 10.1021/acscatal.6b01668 ACS Catal. 2016, 6, 6948−6958
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ACS Catalysis Table 1. Catalytic Performances of Carbon Materials in Aerobic Oxidationa of Cyclohexane68
selectivitiese catalyst g
blank μ-D nano-D G325 G4000 r-GO r-GOh meso-G meso-Gi meso-Gj
X [%]
rw [mmol ·g ·h ]
rs [mmol ·m ·h ]
KA
CyOOH
AA
GA
TOF [h−1]f
2.2 4.4 8.8 6.2 7.4 17.3 15.8 22.1 21.1 21.2
− 30.7 61.3 43.4 51.6 151.6 − 162.6 − −
− 14.4 0.2 12.2 6.2 0.5 − 0.1 − −
64.8 31.4 40.3 60.1 67.7 39.3 36.7 34.4 35.6 32.4
10.3 32.0 27.7 8.5 3.3 2.4 1.8 1.6 3.1 1.6
8.1 6.3 9.1 24.5 20.8 48.1 50.1 53.9 50.6 49.6
− 8.8 6.9 5.0 4.7 3.4 6.2 2.3 4.5 4.3
− 304.7 4.3 202.8 111.0 9.4 − 1.7 − −
b
−1
−1 c
−2
−1 d
a
Reaction conditions: 398 K, 1.5 MPa O2, 1100 rpm, 14.0 g of CyH, 0.39 g of QO, 9.6 g of acetone, and 30 mg of catalyst. bCyH conversion at 8 h. cInitial reaction rate of CyH consumption normalized by catalyst mass. dInitial reaction rate of CyH consumption normalized by catalyst surface (conversion 90% yield of epoxide from stilbene was obtained using tert-butyl hydroperoxide as the oxidant. A linear correlation between the stilbene epoxide yield and the amount of graphitic N atoms in the catalyst was found, which suggests that these graphitic nitrogen atoms played a significant role in this reaction. Amphiprotic buffers such as NaHCO3 and urea were used to control the decomposition of oxidants to complete this selective oxidation reaction. Further DFT simulations revealed that for these oxygen atoms in the catalytic cycle, the α-oxygen (not bound to hydrogen) would react with stilbene while the β-oxygen (bound to hydrogen) would hop to another carbon atom adjacent to the graphitic N atom.70 Both of these catalytic systems employ tert-butyl hydroperoxide as the oxidant, and it is important to control the decomposition rate of the limited amounts of oxidants.71 It is obvious that a poor activation capability of tert-butyl hydroperoxide leads to a low conversion of substrate and also a low yield of oxidation products. On the contrary, a higher decomposition rate of tert-butyl hydroperoxide, which is far beyond the rate of activation of the substrates, results in the conversion of oxidants to inert oxygen instead of active oxygen species that can oxidize the substrate. The activation mismatch of the oxidant and the substrate will lead to a drop in the efficiency of the oxidant and the yield of oxidation products. Besides the benzylic activation process, such N-doped graphene materials can be expanded to other substrates. In fact, this catalyst system could utilize oxygen as the oxidant to oxidize ethylbenzene with the assistance of radical initiators.37 Wang and co-workers reported the use of N-doped graphene to catalyze the oxidation of benzylic alcohols to the corresponding benzaldehydes by oxygen (1 MPa). The conversions of the alcohols were quite low (
